1. Field of the Invention
The invention relates to rotating data transmission devices for transmitting digital signals between a plurality of units which are rotatable with respect to each other.
For the sake of clarity no distinction will be made in this document between a transmission between units that are movable relative to each other, and a transmission between a fixed unit and units movable with respect thereto, because this is only a matter of reference to location and does not affect the manner of operation of the invention. In the same way, no distinction is made between a transmission of signals and energy, because here the mechanisms of operation are the same.
2. Prior Art
With rotatable units such as radar installations or computer tomographs, and also with linearly movable units such as crane and conveyor systems, it is necessary to transmit electrical signals or energy between units movable relative to each other. For this, usually a conductor structure is provided in a first unit, and a corresponding tap in a second unit. In the following exposition the term conductor structures relates to all conceivable forms of conductor structures which are suitable for carrying electrical signals. It also relates to known contacting sliding tracks or slip rings. Of importance to a transmission by means of rotating data transmission devices or linear “slide contact lines” that may also be designed to be non-contacting, is the small distance of the transmission between the units that are movable relative to each other. Thus, a signal may be coupled out optionally by electrical contact, or within a near field of the conductor structures.
A device corresponding to this is described in the German Laid Open Print DE 44 12 958 A1. Here a signal to be transmitted is fed into a strip line on the first unit, that is disposed alongside a path of movement of the units which are movable relative to each other. The signal is tapped off from the second unit by means of capacitive or inductive coupling.
The coupling factor of a signal between the two units depends substantially on the distance of the two units from each other. Particularly for spatially extended transmission systems and especially at high speeds of movement, the distances between the movable units cannot be ascertained with any desired accuracy owing to mechanical tolerances. In practice the distances may vary in a range from direct contact up to a few centimeters, preferably between 0.01 mm and 10 mm. Therefore the coupling factor will frequently vary with the position of the two units with respect to each other, the speed (e.g. by causing vibrations), and other parameters of influence. At the same time the signal amplitude at the input of the receiver varies. This results in changes of the signal with conventionally constructed receivers, which will appear, for example, as an increased jitter, or even bit errors. Similarly, changes of the resistance to interference result.
An improvement of the transmission properties is provided by a device published in DE 197 00 110 A1, which has a conductor structure with filter properties, instead of a strip transmission line. Basically, however, the problems remain.
In U.S. Pat. No. 6,433,631 B2 a device for regulating the input level at a receiver is disclosed. For this, the signal amplitude outputted by a preamplifier is measured, and an attenuating member provided to precede the preamplifier is controlled according to this signal amplitude. The disadvantage of this arrangement is that with it only a signal having a constant amplitude is placed at the disposal of the receiver.
The disadvantage of the devices corresponding to prior art resides in an as yet inadequate resistance to interference. Thus, the transmitted signal levels in the line can be increased in order to improve the resistance to interference. With this, however, an undesired radiation of high-frequency signals increases. With a reduction of the transmitted signal levels, the radiation becomes less, but the immunity to interference scattered in from the outside also becomes less.